Magnet-free optical circulator routes light on photonic chips

May 11, 2018 //
By Julien Happich

Researchers from the FOM Institute AMOLF and the University of Texas have demonstrated a micrometer-sized optical circulator consisting of a silica microtoroid to which tapered optical fibres feed incoming light beams.

In a paper titled "Optical circulation in a multimode optomechanical resonator" published in Nature Communications, the researchers highlight that for the first time, an optical circulator was designed "magnet-free", relying on tiny mechanical vibrations imparted to a glass ring to directionally route light on an optical chip.

“Light propagation is symmetric in nature, which means if light can propagate from A to B, the reverse path is equally possible. We need a trick to break the symmetry”, explains AMOLF group leader Ewold Verhagen. “Usually this ‘trick’ is using centimetre-sized magnets to impart directionality and break the symmetric nature of light propagation. Such systems are difficult to miniaturize for use on photonic chips.”

Artist impression of the light circulator. The yellow beam
enters at the upper left port and is forced to leave the
resonator at the lower left port. The red beam enters at that
port (lower left) but cannot follow the reverse path of the
yellow beam as it is forced to propagate to the lower right exit.
Credits: Henk-Jan Boluijt (AMOLF).

Only a few micrometers across, the novel silica circulator allows light in the ring to interact with mechanical vibrations of the same structure, hence routing light uni-directionally between several ports.

“By shining light of a ‘control’ laser in the ring, light of a different color can excite vibrations through a force known as radiation pressure, but only if it propagates in the same direction as the control light wave”, Verhagen explains. “Since light propagates differently through a vibrating structure than through a structure that is standing still, the optical force breaks symmetry in the same way as a magnetic field would.”